Various anatomical exercise and therapy devices for exercising or conducting specific therapy movements of different muscle groups of a patient are well known in the art. As an example, continuous motion passive exercise machines have now become the standard of care for rehabilitation of joint injuries of injured or surgical patients.
In general, a passive motion exerciser moves a body part such as an arm or leg through a range of motion. This simulates the operation of the muscles and joints associated with the body part. Such passive motion exercisers may be continuous in motion and driven by electric motors or other continuous drive means. U.S. Pat. No. 4,355,633 to Heilbrun discloses such a passive exerciser apparatus that is motor driven.
Such passive exercise devices are useful for rehabilitating shoulder joint injuries or for rehabilitation following surgery of the shoulder, arms, or neck of a patient. A problem with such passive exercise devices, as related to shoulder joint rehabilitation, is that the prior art devices do not compensate for the anatomical movement of the shoulder joint and muscles during flexion or abduction of the shoulder.
As shown in FIG. 1, the shoulder joint structure includes a gleno humeral or glenoid joint which provides articulation for the humerus (i.e. upper arm bone) with respect to the flat triangular scapula (i.e. shoulder blade). Movement of the arm by various arm or shoulder muscles may universally rotate the humeral head within the glenoid joint as indicated by arrows 10. This is referred to as gleno-humeral motion.
In addition to gleno-humeral motion, the shoulder joint also undergoes scapula-thoracic motion. As shown in FIG. 1, the scapula lies within the dorsal lateral part of the thorax and is articulated with the clavicle. The scapula may be displaced from its position within the thorax by various arm and shoulder muscles as indicated by arrows 12. This is referred to as scapula thoracic motion. Rotation of the scapula accounts for about one third of total shoulder motion.
As a consequence of this shoulder joint structure, different movements of the shoulders and arms during exercise, produce different relative locations for the glenoid joint. These movements may include flexion and extension of the shoulder, shoulder flexion and abduction, and shoulder internal rotation. During each of these movements the center of rotation of the glenoid joint may change or shift within the shoulder. Prior art passive exercise devices typically account for gleno-humeral motion of the glenoid joint structure (i.e., universal rotation as indicated by arrows 10) but not scapula-thoracic motion of the joint structure (i.e. rotation of the scapula within the thorax as indicated by arrows 12). These prior art devices therefore apply a constant force to a fixed center of rotation. This does not accommodate the changing center of rotation of the shoulder joint caused by the combined gleno-humeral and scapulathoracic motions.
FIG. 2 illustrates the movement of the center of rotation of the shoulder joint (i.e., glenoid joint) during flexion of the shoulder. In FIG. 2 a patient 14 may flex his arm 16 through a range of motion from 0.degree. to 180.degree.. During flexion, in addition to the gleno-humeral motion of the glenoid joint, the joint also rises superiorly and rotates posteriorly due to scapula-thoracic motion. This motion changes the location of the center of rotation of the joint. For 50.degree. flexion, a center of rotation is indicated by 18. For 180.degree. flexion a center of rotation is indicated by 18'. Likewise, as shown in FIG. 3, during abduction of a patient's arm 16, from 50.degree. to 180.degree., the center of rotation 18 may be shifted as indicated to 18'.
Since prior art exercise devices typically do not compensate for the relative movement of the glenoid joint caused by this scapula-thoracic motion, undue stresses may be induced by the arm and joint being held at a fixed center of rotation by points of attachment with the exerciser device. This may cause the patient to experience pain and discomfort and decrease the length and effectiveness of exercise. Additionally it may cause damage to the already injured shoulder joint and muscles. This problem may be further compounded if the patient changes position during exercise relative to the exercise device.
The shoulder exerciser of the invention, on the other hand, is constructed to allow the shoulder joint and muscles to follow a natural anatomic range of motion during flexion and abduction of the shoulder. The exerciser compensates for the changing center of rotation of the shoulder joint encountered during flexion and abduction of the shoulder through a 180.degree. range of motion. This helps to prevent "jamming" or "stretching" of the glenoid joint and shoulder muscles, and helps minimizes painful stresses on the joint and muscles. Moreover, a patient may change his position, or the position of his arm relative to the exercise device of the invention, without the introduction of stress and discomfort in the glenoid joint and shoulder muscles.